1. Field of the Invention
This invention relates to a process for modifying the surfaces of hard materials, and to cutting tools.
2. Description of the Related Art
Metal cutting tools are partly formed of hard materials including ultrahard alloys, cermets and ceramics. For improving their wear resistance, the surfaces of the hard material portions are coated with a layer or layers formed by chemical or physical vapor deposition (CVD or PVD) from titanium (Ti), hafnium (Hf) or zirconium (Zr) carbide (e.g. TiC), or nitride (e.g. TiN), or carbide-nitride (e.g. Ti(C,N)), or aluminum (Al) oxide (e.g. Al.sub.2 O.sub.3).
Research has also been conducted for developing a special base metal with a surface containing a reduced amount of cobalt (Co) as a result of its exposure to a plasma and exhibiting an improved adhesion relative to a coating film, so that the coating film may have a reduced internal stress and may not peel off easily.
Both known technologies are concerned with the formation of a hard coating film on the base metal. The coating film is, however, greatly different from the base metal in physical and chemical properties, particularly coefficient of thermal expansion, and is, therefore, very likely to crack and peel off the base metal during cooling after vapor deposition at a high temperature. Thus, the film is not satisfactory in terms of durability, though it may be of high hardness and wear resistance. Moreover, the cleansing of the base metal, which has to be done prior to its coating, requires a high level of technique and a long period of time, thereby adding a great deal to the cost of manufacture.